This invention was made with government support under Office of Naval Research Grant No. N00014-94-1-0592 and University of Pittsburgh Material Research Center through the Air Force Office of Scientific Research Grant No. AFOSR-91-0441. The government has certain rights in the invention.
1. Field of the Invention
The present invention generally relates to optical devices and methods for making the same. More specifically, the present invention relates to novel, mesoscopically periodic materials that combine crystalline colloidal array (CCA) self-assembly with the temperature induced volume phase transitions of materials that undergo a volume change in response to temperature changes. These materials are used to create tunable optical devices such as optical switches, optical limiters and optical filters that select and/or reject predetermined wavelengths of light. In addition, these materials can be used to create various display devices and processing elements as well as filtering devices whose pore sizes can be varied.
2. Background Art
Charged colloidal particles, when suspended in water, form a stable dispersion due to interparticle coulomb repulsion forces. The property of structural ordering in such dispersions has been exploited in making devices such as narrow band optical rejection filters. The ordering phenomena in such colloidal suspensions has been useful in spectroscopy and Bragg diffraction techniques. See, for example, U.S. Pat. No. 4,627,689. It has been found that mesoscopic, crystalline structures can have many practical applications as optical filters in military, space, medical and research uses. In many such instances, it is necessary or desirable to filter narrow bands of selected wavelengths from a broader spectrum of incident radiation.
Asher, U.S. Pat. No. 4,627,689 discloses a linear crystalline colloidal narrow band radiation filter which is made by forming a highly ordered crystalline colloidal structure within a container. The crystalline colloidal structure is formed by dispersing the ionized particles, for example, polystyrene particles, within an appropriate solvent.
A related disclosure was made in Asher, U.S. Pat. No. 4,632,517. That patent discloses another crystalline colloidal narrow band radiation filter application which forms the basis for a mechanically simple and highly efficient monochromator. It has application in improved systems for investigating Raman or emission spectra of selected sample materials. Both of the aforementioned patents disclose structures that can be used to diffract a narrow band of radiation from a broader band of radiation.
A solid filter and method of making a solid filter from an ordered dispersion of particles within a medium is disclosed in Asher, U.S. Pat. No.5,281,370. That patent discloses a filter which is capable of Bragg diffracting narrow bands of radiation. It is a solid filter which has many practical applications.
Other filtering devices are also known. For example, U.S. Pat. No. 4,803,688 discloses the use of an ordered colloidal suspension for an optical device.
An optical filter was also disclosed in U.S. Pat. No. 4,548,473. The filter comprises a first substance substantially transparent to light within a select frequency range and having a first index of refraction. The filter also includes a second substance which has at least one resonance frequency within the first frequency range and a second index of refraction which is substantially the same as the first index of refraction at all of the frequencies within the first frequency range except for frequencies near the resonance frequency. This device is based upon resonance scattering by a disordered sample. The device is only a passive device meaning that the index of refraction is not considered to depend upon the incident intensity or time.
U.S. Pat. No. 3,620,597 discloses a device which is capable of acting as a nonlinear absorber of substantially all radiant energy in excess of a predetermined intensity. The mechanism utilized by the device is distinct from that of the present invention.
U.S. Pat. No. 4,832,466 discloses an optical element including a modulating liquid layer composed of a solvent containing a soluble polymer. The device requires polymers to precipitate from solution due to temperature changes. This is not required by the present invention.
U.S. Pat. No. 4,648,686 discloses an optical switch array which utilizes the temperature dependent characteristics of the index of refraction of a crystalline material, however, the device is limited to being used for switching in a waveguide. Other switches for use in waveguides were disclosed in U.S. Pat. Nos. 4,828,362 and 4,938,557.
U.S. Pat. No. 4,268,413 discloses devices having the property of reversibly variable temperature-light absorbance. The device is said to be usable in temperature-measuring devices, slippery ice warning devices and the like.
U.S. Pat. No. 5,452,123 discloses a nonlinear optical device and method for making the same. The method includes making a solid or crystalline colloidal ordered dispersion of charged particles within a medium and introducing into the particles or the medium a radiation responsive component which, when impinged with radiation at a critical density, causes a change in the refractive index of the particles in either the ordered dispersion, the medium or both.
U.S. Pat. Nos. 5,368,781 and 5,266,238 are directed to tunable, narrow band radiation filters comprising a crystalline colloidal array of charged particles fixed in a hydrogel film. Methods for filtering incident radiation using these filters are also disclosed.
U.S. Pat. No. 4,720,355 is directed to a non-linear optical medium having a "host" thermoplastic polymer which contains a "guest" organic component; the organic component has a charge asymmetric electronic structure and exhibits non-linear optical response.
U.S. Pat. Nos. 5,330,685, 5,338,492 and 5,342,552 are all directed to narrow band radiation filters comprising a CCA of charged particles in a polymeric hydrogel.
None of the above patents disclose the unique devices of the present invention. There remains a need, therefore, for optical devices that diffract a narrow predetermined wavelength band and are easily tunable in terms of diffraction efficiency and the wavelength region diffracted.